There is a pressing need to increase the amount and/or different types of information that communication and navigation systems can transmit, without appreciably degrading the frequency spectrum associated with the transmitted information. For example, widening of the frequency spectrum and shallowing of frequency nulls within the frequency spectrum should be avoided. Such information includes, but is not limited to authentication information. Authentication may be used to identify equipment, such as transmitters and receivers, and various signals used in the communication and navigation systems. Authentication methods have been proposed for Global Positioning System (GPS) signals to combat “spoofing” attacks that inject false signals. The authentication methods enable a GPS receiver to determine that a GPS navigational waveform received by the receiver is legitimate. One such method proposes to overlay a known “watermark message” on the GPS navigational waveform transmitted from a space vehicle or other GPS broadcast platform. Such watermarking must not impact compliance with the baseline GPS navigational waveform. Methods have also been proposed to overlay an additional data channel or watermark onto a “host” data communication link. Whether the host is a navigational waveform or a data communication channel, the overlaid watermark or data channel should cause minimal degradation to the demodulated host waveform performance while providing enough energy to communicate the watermark or data message in weak signal or challenged environments such as urban multipath and jamming. The watermarking should minimize impacts on both transmitter and receiver complexity. Conventional approaches to provide such watermark overlays for GPS and general data links meet these criteria only to a limited extent in that there is still significant degradation to the underlying host waveform in order to provide a robust watermark or data overlay.